1. Field of the Invention
The present invention relates to a process for producing porous thermoplastic article in the form of hollow yarn or film.
2. Description of the Prior Arts
A porous hollow yarn made from a polymer material having the surrounding wall with a large number of micropores formed therein is used widely as a device for separating various substances. For example, in the clinical field, it is employable for separation of blood plasma and ultrafilteration.
As a process of producing a porous hollow yarn, there is known a process which comprises the steps of preparing a hollow yarn from a polymer material containing a soluble material dispersed therein and then removing said soluble material by dissolving it with an appropriate solvent to form a large number of micropores in the surrounding wall of said yarn. Recently, another process using a hollow yarn of a thermoplastic crystalline polymer material has been proposed and now is utilized in practice. By this process, said hollow yarn is heat-treated and then subjected to a drawing process to form pores in the surrounding wall. Thermoplastic polymers which can be used for the above purpose are polyolefin, polyamide, polyester and other analogous copolymers. Among those materials, polypropylene (homopolymer of propylene or copolymer of propylene and other monomers) is considered to be satisfactory as the polymer material for producing a porous hollow yarn because it is excellent in moldability, mechanical strength and resistance to chemicals.
Processes of producing a porous hollow yarn using polypropylene as the polymer material have been described in Japanese Patent Provisional Publications No. 52(1977)-15627, No. 52(1977)-137026, No. 53(1978)-38715, No. 54(1978)-34418, No. 54(1979)-68414, No. 54(1979)-120735, No. 54(1979)-138623, No. 55(1980)-1314, No. 55(1980)-5914, etc. Most of these processes disclosed in the above publications comprise the main procedure in which a polypropylene hollow yarn spinned is first heat-treated and then drawn in the vicinity of room temperature to form micropores so as to produce a porous body, and the resultant body is again heat-treated so as to have micropores fixed therein.
In view of the application purpose of a porous polypropylene hollow yarn, micropores formed in the surrounding wall are desired to be uniformly distributed at a density as desired (the density is expressed by the void ratio). The process to produce such a yarn of desired properties has been disclosed in Japanese Patent Provisional Publications No. 54(1979)-34418, No. 54(1979)-68414, 54(1979)-138623, etc. in which the degree of crystal orientation in a yarn is enhanced by heat treatment under specific conditions, and then the yarn is drawn to form micropores therein. Another process which improves the above process by eliminating such a complex treatment has been disclosed in Japanese Patent Provisional Publication No. 55(1980)-1314, in which the degree of crystal orientation in a yarn is enhanced by setting the conditions of spinning (which is done prior to the drawing process of a polypropylene hollow yarn) within specific ranges.
In the above-described conventional process, the step of enhancing the crystal orientation in a polypropylene hollow yarn is necessarily introduced before the drawing step in order to give the desired property to a porous polypropylene hollow yarn. Therefore, the manufacturing process as a whole still remains to be complicated.
Also known is a porous thermoplastic film having a large number of micropores formed therein. Such porous film is used widely, for instance, as a filtering or separating device for air cleaning or water treatment, a diaphragm for electric cell or electrolysis, or a separating membrane for artificial lung or blood plasma.
As a process of producing the porous thermoplastic film, there can be employed the essentially same process as described in the preparation of the porous hollow yarn.
Various thermoplastic, crystalline polymers employable for the above purpose are known. Particularly, polyolefin, fluorine-containing polymer, polyamide, polyester and other analogous copolymers are generally used. Among those materials, polypropylenes (homopolymer of propylenes or copolymers of propylene and other monomers) and fluorine-containing polymers are considered to be satisfactory as the polymer materials for producing a porous thermoplastic film because it is excellent in mechanical strength and resistance to chemicals.
Processes of producing a porous thermoplastic resin films have been described, for instance, in Japanese Patent Publication No. 46(1971)-40119, No. 50(1975)-2176, No. 55(1980)-32531, and No. 54(1979)-34418. Most of these processes disclosed in the above publications comprise the main procedure in which a thermoplastic resin film is first heat-treated and then drawn at the vicinity of room temperature or at a temperature of not lower than the second order transition point of the thermoplastic resin employed (e.g., not lower than -40.degree. C. in the case that a polypropylene is employed as the resin) to form micropores producing a porous body, and the resultant body is again heat-treated so as to have micropores fixed therein.
In view of the application purpose of a porous thermoplastic resin film, micropores formed in the film are desired to be uniformly distributed at a density as desired (the density is also expressed by the void ratio). However, the porous thermoplastic film obtained in the above process has a relatively small average pore size such as not larger than 5,000 angstrom and low void ratio. For these reasons, the porous thermoplastic film obtained in the above process is not appropriate in the use for membrane for separation of blood plasma, etc., and the use of the porous film is thus under limitation.
It is known that a porous thermoplastic resin film having uniformly distributed, relatively large micropores and a high void ratio can be produced by the above-stated known method using a thermoplastic film having high orientation degree or high elastic recovery. Such thermoplastic films can be produced by forming the film under specific conditions, or by enhancing the crystal orientation degree through heat-treatment of the resin film. Thus, the known methods generally require a supplemental procedure for enhancing the crystal orientation degree of the resin film, if a porous thermoplastic resin film having high quality is desired. The process for producing a porous thermoplastic film in which the supplemental procedure is introduced is relatively more complicated as a whole.